This invention relates generally to centrifugal compressors and, more particularly, to a method and apparatus for providing a seal between an oil-fed transmission chamber and the relatively low pressure area in a balance piston adjacent the impeller.
In order to counteract the aerodynamic thrust that is developed by the impeller of a centrifugal compressor, it is well known to employ a balance piston consisting of a low pressure cavity behind the impeller wheel. Because of the tendency for lubricating oil to leak from the transmission into this low pressure area, it is also common practice to install a seal device between the balance piston and the transmission. A mechanical seal, such as a carbon face seal, is typically used for this purpose. However, besides being very intricate, delicate and expensive, these mechanical seals introduce substantial mechanical losses due to viscous drag from relative motion between mating surfaces.
An alternative is a labyrinth seal which is simple, rugged, inexpensive and, since it is noncontacting, there is virtually no mechanical losses due to rubbing. The disadvantage, however, is that in order to be entirely effective, it is necessary to pressurize the labyrinth seal. One known way to do so in a centrifugal compressor is to fluidly connect a source of high pressure gas from the discharge line to the center of the labyrinth. In this way, oil leakage from the transmission is substantially eliminated.
A disadvantage of such a pressurized labyrinth seal as recognized by the Applicants is that the high pressure gas at the labyrinth will tend to flow into the balance piston and the transmission chamber, and if the flow becomes excessive, the overall efficiency of the compressor will suffer. Further, the flow into the balance piston will tend to degrade its performance.
In particular, with regard to efficiency losses in higher pressure systems, such as a centrifugal compressor designed for an operation with a high density refrigerant such as R-22, the pressure differential between the compressor discharge line and the transmission, and even more so, the pressure differential between the discharge line and the balance piston, can be sufficiently high that there will be a substantial flow of refrigerant gas to the balance piston and into the transmission. The transmission is vented by means of a pipe back to compressor suction. Also, the balance piston is ported to compressor suction. Thus, any high pressure gas leaking in either direction ends up being re-compressed and is therefore cause for a loss in efficiency.
It is therefore an object of the present invention to provide an improved labyrinth seal arrangement for a centrifugal compressor.
Another object of the present invention is the provision in a centrifugal compressor for the effective and efficient use of a balance piston.
Yet another object of the present invention is the provision in a centrifugal compressor for maintaining an effective and efficient seal between the transmission and a low pressure cavity of a balance piston structure.
Still another object of the present invention is the provision in a centrifugal compressor for reducing the leakage of high pressure labyrinth seal gas to a balance piston cavity.
Yet another object of the present invention is the provision in a centrifugal compressor for a labyrinth seal arrangement which is economical to manufacture and reliable and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.